What is the right way of handling python-social-auth exceptions when using Flask?

I'm adding PSU to a Flask app and all is going pretty well so far, but I can't figure out how to handle the exceptions raised by PSU. One such exception is social.exceptions.AuthCanceled, raised when a user decides to cancel the auth process. I would obviously want to catch that and display some message.

I found a similar question on how to do this in Django by creating a new Middleware. However, that approach seems to use middleware.py defined only in PSU's django_app (and not in the flask_app).

I have some experience with Flask but haven't added Middleware before, and I'm not sure this is the right direction.

Solution

UPDATE

Try defining an errorhandler (docs at http://flask.pocoo.org/docs/api/#flask.Flask.errorhandler), like this:

@app.errorhandler(500)  # since exceptions will produce 500 errors
def error_handler(error):
    if isinstance(error, SocialAuthBaseException):
        return redirect('/error')

The solution below this line won't work

Try with a teardown_request (http://flask.pocoo.org/docs/reqcontext/#teardown-callbacks), like this

@app.teardown_request
def teardown_handler(exception=None):
    if exception and isinstance(exception, SocialAuthBaseException):
        return redirect('/error')